1. Field of the Invention
The present invention relates to a method for producing a semiconductor laser device. In particular, the present invention relates to a method for producing a distributed feedback semiconductor laser device which is capable of stable control of the transverse mode to obtain a fundamental transverse mode even at a high output, and therefore is suitable for a light source for an optical data processing or an optical measuring instrument.
2. Description of the Prior Art
Recently, a semiconductor laser having the advantages of compactness, high output, and low cost has been put into practical use and applied to general industrial equipment and consumer equipment in which it has been difficult to use a conventional laser as a light source. The application of the semiconductor laser is especially remarkably advanced in the fields of optical data processing, optical measuring, and optical communication. It is expected that a semiconductor laser will be applied in more diverse fields in the future. Under these circumstances, it is demanded to further improve the inherent features of the laser such as coherency and monochromaticity of light and their stability, and thus to develop a semiconductor laser device having characteristics which are close to those of a gas laser. For example, a semiconductor laser device for application in a high speed laser printer has been developed.
For the application in a laser printer, a laser in which the transverse mode is controlled to obtain the fundamental transverse mode is required. In the optical system using a hologram or the like, a semiconductor laser, in which the transverse mode is controlled to obtain the fundamental transverse mode and the longitudinal mode is controlled to obtain the single longitudinal mode, is required. In response to these demands, a semiconductor laser device is proposed which has a diffraction grating formed in a resonator and thus selectively controls only the longitudinal mode corresponding to the Bragg wavelength of the diffraction grating. Such a semiconductor laser device is called a distributed feedback semiconductor laser device (hereinafter, referred to as DFB-LD). GaAs or InP semiconductor laser devices having such a construction have been developed.
Today, a semiconductor laser device using InP/InGaAsP materials and having a wavelength of 1.3 .mu.m or 1.55 .mu.m is practically used in a part of the optical communication field. The DFB-LD structure is employed in an AlGaAs/GaAs semiconductor laser device having a wavelength of 830 nm or shorter for use as a stable light source in a system utilizing a hologram or diffraction grating, for example, a hologram laser beam scanner and an integrated pickup for an optical disk.
A method for producing a DFB semiconductor laser device which includes a stripe groove and a light absorbing current block layer portions disposed on both sides of the stripe groove and thus controls the transverse mode is disclosed in Japanese Laid-Open Patent Publication No. 2-206191. According to this publication, a diffraction grating is formed in the semiconductor laser device in a simple method including only two crystal growth steps. This semiconductor laser device, which has a schematic construction shown in FIG. 7, is produced in the following manner.
In a first crystal growth step, as is shown in FIG. 8, an n-In.sub.0.5 (Ga.sub.0.3 Al.sub.0.7).sub.0.5 P first cladding layer 31, a non-doped In.sub.0.5 Ga.sub.0.5 P active layer 32, a p-In.sub.0.5 (Ga.sub.0.7 Al.sub.0.3).sub.0.5 P optical waveguide layer 33, and an n-GaAs current block layer 34 are sequentially grown on an n-GaAs substrate 30 under a reduced-pressure by use of metal organic chemical vapor deposition (hereinafter, referred to as MOCVD).
Next, as is shown in FIG. 9, the current block layer 34 is etched by use of a chemical etching method, applying a photo resist mask until the optical waveguide layer 33 is exposed, thereby forming a stripe groove.
After the photo resist mask is removed by an appropriate solvent, the optical waveguide layer 33 inside the stripe groove is applied with another photo resist mask. A periodical corrugation is formed at the photo resist mask by use of holographic exposure, and then the periodical corrugation is transferred onto a surface of the optical waveguide layer 33 by use of a chemical etching method through this photo resist mask, thereby forming a diffraction grating 37 (FIG. 7).
In a second crystal growth step, as is shown in FIG. 10, a p-In.sub.0.5 (Ga.sub.0.3 Al.sub.0.7).sub.0.5 P second cladding layer 35 is grown on the optical waveguide layer 33 in the stripe groove and on the current block layer 34, and a p-GaAs contact layer 36 is grown on the second cladding layer 35, both by use of MOCVD.
Finally, a p-type electrode 38 is formed on a top surface of the contact layer 36, and an n-type electrode 39 is formed on a bottom surface of the substrate 30, and then the obtained wafer is cleaved into chips to produce the DFB semiconductor laser device shown in FIG. 7.
In the above DFB semiconductor laser device, portions of the light absorbing current block layer 34 disposed on both sides of the stripe groove controls the transverse mode. The longitudinal mode is controlled by the diffraction grating 37.
The above method has an advantage of producing the semiconductor laser device in only two crystal growth steps, but has the following problems.
It is extremely difficult to uniformly apply photo resist to form the photo resist mask for forming the diffraction grating on a bottom of the narrow groove sandwiched between the thick current block layers 34. For example, the photo resist tends to be thickly applied in the vicinity of the current block layer at the bottom of the groove. This portion is also hard to expose because the current block layer acts as a shielding wall, so the resist is not irradiated with UV light. Accordingly, it is impossible to form the diffraction grating in the vicinity of the current block layer on the bottom of the groove. In order to form a uniform diffraction grating at an entire bottom area of the stripe groove, the stripe groove acting as the waveguide region should be formed so as to have a width as wide as 10 .mu.m or more. Such a wide waveguide region causes a deformation in the transverse mode due to spatial hole burning in the waveguide region, thereby adversely affecting the characteristics of the semiconductor laser device.
In the semiconductor laser device produced in the above method, the waveguide region in the vicinity of the current block layer without diffraction grating is thicker than the waveguide region at the central bottom portion of the stripe groove. Accordingly, the former portion has a larger equivalent refractive index than the latter portion. Since light is led to a portion having a larger refractive index, the former portion has a larger optical intensity distribution than the latter portion. As a result, the loss of light due to the light absorption effect of the current block layer is large, thereby excessively losing the light in the waveguide region in the fundamental transverse mode.
For these problems, the semiconductor laser device produced in the above prior art method is not always excellent in quality, resulting in a low production yield.
FIG. 21 shows a typical conventional AlGaAs/GaAs DFB semiconductor laser device having a mesa ridge and portions of a light absorbing current block layer disposed on both sides of the mesa ridge as a transverse mode controlling structure. This semiconductor laser device is produced, for example, in two crystal growth steps by use of MOCVD. An optical waveguide layer necessary for laser emission and a diffraction grating important for the DFB-LD structure are formed between the two crystal growth steps. Practically, this semiconductor laser device is produced in the following manner.
As is shown in FIG. 22, an n-AlGaAs first cladding layer 401 (thickness: 1.3 .mu.m), an AlGaAs active layer 402 (thickness: 0.12 .mu.m), and a p-AlGaAs optical waveguide layer 403 (thickness: 0.3 .mu.m) are sequentially grown on an (100) facet of a n-GaAs substrate 400 by use of MOCVD. The resulting wafer is then taken out of a liquid phase epitaxy (hereinafter, referred to as LPE) apparatus, and a surface of the optical waveguide layer 403 is etched by an appropriate etching method to form a mesa ridge 404. The mesa ridge 404 has, for example, a width of approximately 3 .mu.m and a height of approximately 0.15 .mu.m. The mesa ridge 404 constitutes an optical waveguide region. As is shown in FIG. 23, a diffraction grating 405 is formed at an entire top surface of the optical waveguide layer 403 having the mesa ridge 404 by use of holographic exposure.
A p-AlGaAs second cladding layer 406 (thickness: 1.0 .mu.m) and a p-GaAs contact layer 407 (thickness: 0.3 .mu.m) are grown on the diffraction grating 405 by use of MOCVD. Then, a SiO.sub.2 film 408 (thickness: 0.2 .mu.m) is formed on the contact layer 407 by use of plasma chemical vapor deposition (P-CVD). A ortion of the SiO.sub.2 film 408 positioned right above the mesa ridge 404 is etched away to form a window in a stripe pattern 409 (width: 3 .mu.m). The current injection path is regulated by the stripe pattern.
Finally, a p-type ohmic electrode (not shown) is formed on a top surface of the contact layer 407 in the window 409, and an n-type ohmic electrode (not shown) is formed on a bottom surface of the substrate 400, and the resulting wafer is cleaved into chips to produce the DFB semiconductor laser device shown in FIG. 21.
The above method has the advantage of simplicity owing to only two crystal growth steps required, but has the following problems.
Since the positional alignment of the mesa ridge 404 and the window 409 should be performed so that the gain generated by current injection is largest at the center of the optical waveguide region in the transverse direction, an extremely high precision (generally, an error of 0.1 .mu.m or less) is required. Practically, since the mesa ridge 404 is buried by the second cladding layer 406 and the contact layer 407, such a highly precise alignment is extremely difficult. When the current injection path and the optical waveguide region are even slightly deviated in position from each other, the control of the transverse mode is ineffective. Since an effective refractive index waveguide is utilized in order to control the transverse mode in this semiconductor laser device, a higher order mode or an asymmetrical mode light cannot be reduced. Especially in the high optical output operation, the peak of the emitted laser intensity distribution is easily deviated from the center of the waveguide region.
In forming the diffraction grating 405, it is necessary to apply resist on a surface of the optical waveguide layer 403 on which the mesa ridge 404 is formed and then expose the resist so as to have a stripe pattern. Since the thickness of the resist film is non-uniform at the vicinity of the foot of the mesa ridge 404, it is difficult to determine appropriate exposure conditions. Accordingly, the diffraction grating cannot be formed to have a uniform thickness.
In order to solve these problems, still another method for producing a DFB semiconductor laser in which a diffraction grating is formed on an entire surface of the flat wafer is proposed in Japanese Laid-Open Patent Publication No. 2-206191. The semiconductor laser device according to this publication is shown in FIG. 24, and is produced in the following manner.
An n-InGaAlP first cladding layer 501, an InGaP active layer 502, and a p-InGaAlP optical waveguide layer 503 are sequentially grown on an n-GaAs substrate 500. Next, a diffraction grating 504 is formed at an entire top surface of the optical waveguide layer 503, and then a p-InGaAlP second cladding layer 505 is grown on the diffraction grating 504. The second cladding layer 505 is etched by use of a thin film mask formed of a dielectric material to form a mesa ridge 506. The thin film mask formed of the dielectric material is used again to grow a current block layer 507 on the optical waveguide layer 503 provided on both sides of the mesa ridge 506. Then, the dielectric thin film mask is removed, and a p-GaAs contact layer 508 is grown on the mesa ridge 506 and the current block layer 507. Finally, a p-type ohmic electrode 509 is formed on a top surface of the contact layer 508, and an n-type ohmic electrode 510 is formed on a bottom surface of the substrate 500. Thus, the DFB semiconductor laser device shown in FIG. 24 is produced. In this prior art example, all the layers are grown by use of MOCVD.
In the DFB semiconductor laser device having the mesa ridge shown in FIG. 24, the diffraction grating is formed at the entire surface of the optical waveguide layer. Therefore, this semiconductor laser device is more excellent than the one in FIG. 7 in the control precision of the manufacturing process. On the other hands, this semiconductor laser device has problems that a dielectric film should be used as a mask to selectively grow the current block layer and that four crystal growth steps are required to grow the contact layer, resulting in a low production yield.